1. Field of the Invention
This invention relates to a process for the production of ether carboxylic acid salts by oxidation of ether alcohols with oxygen in the presence of a catalyst.
2. Description of the Related Art
The production of ether carboxylic acids and salts thereof is described in a number of publications. For example, U.S. Pat. No. 4,214,101 describes a process for the production of ether carboxylic acids in which fatty alcohol polyglycol ethers are contacted while stirring with oxygen in the presence of a palladium catalyst at a pH value of 8 to 13 and at a temperature of 50.degree. to 95.degree. C. On completion of the reaction, the catalyst is filtered off and the free ether carboxylic acid is released from the alkali metal salt by acidification.
Patent applications DE-A 28 16 127, DE-A 34 46 561 and EP-A 0 073 545 describe similar processes, namely oxidation with pure oxygen under slight excess pressures (above atmospheric pressure) and with catalysts, such as Pd/C, Pt/C, or mixed catalysts, such as Pd/Pt/C, the reaction being carried out at a pH value of 8 to 13.
One feature common to the processes described in the documents mentioned above is that the entire ether alcohol is introduced at the beginning of the reaction and the oxidation is carried out under oxygen partial pressures slightly above atmospheric pressure (P.sub.02 approximately 1 bar abs.).
As described in DE-A 28 16 127, it was found when carrying out the reaction that the viscosity of the reaction solution initially increases with increasing conversion, reaching a maximum at a conversion of 20 to 30% and then falling drastically as the reaction progresses. In tests carried out by applicants, it was found that the level of the viscosity maximum increases with increasing concentration of the ether alcohol used, with increasing C chain length and with a decreasing number of ethylene oxide groups. The effect of the viscosity maximum is that the reaction takes place more slowly and, in the most unfavorable case, actually comes to a stop. For example, it is almost impossible to oxidize a 20% aqueous C.sub.12/14 4EO ethoxyalcohol solution to form the corresponding ether carboxylic acid. In this case, viscosity increases to such an extent that the reaction comes to a stop. On the other hand, a C.sub.12/14 11EO ethoxyalcohol of the same concentration can be oxidized without problems. For the reasons mentioned above, however, it is not possible to react an only 10% aqueous C.sub.16/18 6EO ethoxyalcohol solution.
DE-A 39 29 063 describes a process in which the viscosity can be kept below the maximum by introduction of the ether alcohol during the reaction in dependence upon its consumption. A solution with a high concentration of reacted ether alcohol can thus be obtained on completion of the reaction. Through addition of the ether alcohol at a rate commensurate with that at which it reacts off, the concentration of unreacted ether alcohol in the reaction solution can be kept substantially constant throughout the reaction.
In the process described in DE-A 39 29 063 the oxidation is not carried out with pure oxygen, but instead with air. This inevitably results in a waste gas stream. On account of the intensive foaming of the starting materials and end products, foam is discharged with the waste gas. To suppress foaming and hence the discharge of reaction solution, special equipment-related measures have to be taken, for example by the provision of a co-current column with a suspension circuit such as described in DE-A 39 29 063. A disadvantage of oxidation with air is that the waste gas stream entrains ether alcohol in accordance with its vapor pressure and thus causes a certain degree of environmental pollution.
A disadvantage of carrying out the process under oxygen pressures above atmospheric pressure as described in the prior art is that the catalyst can be deactivated by an oversupply of oxygen. Tests have shown that, in particular, a repeatedly used catalyst is prematurely deactivated where the reaction is carried out under high oxygen pressures so that only an incomplete conversion is obtained.
The problem addressed by the present invention was to provide an industrial-scale process for the production of ether carboxylic acids by oxidation of ether alcohols with oxygen, in which ethoxyalcohols with any ethylene oxide contents and any carbon chain lengths could be oxidized in high conversions by continuous introduction of ether alcohol to form the corresponding ether carboxylic acids or alkali metal salts thereof in the form of highly concentrated solutions, the reaction being carried out without any waste gas using an already repeatedly used catalyst.